1. Field of the Invention
The present invention relates generally to an apparatus for automatically controlling the operation of a door, and more particularly to an apparatus which performs a time related analysis of Doppler signals developed in response to the movement of a moving object and is operative to open the door when such movement is detected.
2. Description of the Prior Art
Automatic doors of either the swinging or sliding type now form entrances to most office building, supermarkets, commercial establishments, and the like. Swinging doors are generally recognized as the industry standard, although sliding doors have achieved strong acceptance in the last decade because of their clean, functional appearance. To automatically open the doors, many door-actuating devices such as floor pressure mats, photo-electric systems and microwave motion sensing devices have been utilized.
Although floor pressure mats were commonly used in the past, architects are reluctant to install them in new buildings because of their unsightly appearance and high maintenance requirements. For example, in northern areas during the snowy season the mats have caused door malfunctions due to snow loading and have experienced deterioration when contacted with chemicals used to melt the snow. In addition, the mat is unable to detect a person standing on the threshold of the door, which could lead to injuries due to the inadvertent closing of the door.
Photo-electric systems which actuate a door when a person walks through a photo-electric beam directed in front of the door have been found to be expensive since several such systems are required to effectively cover the door area.
Heretofore, microwave motion sensing systems have employed a technique for analyzing the amplitude of a sensed microwave signal to actuate a sliding door. Such systems have included a pair of antennas positioned on either side of the door and having respective radiation patterns directed away from the door in a nonoverlapping manner to avoid sensing the sliding motion of the closing door. Since the radiation patterns do not overlap, a zone is formed over the threshold and on either side of the door in which motion cannot be detected. Hence, systems of this type present a potential safety hazard. In addition, the systems are unable to distinguish between man motion and door motion.
It should be noted that motion detection systems functioning on the Doppler principle are available. In one such system generally known as a Doppler-type system, frequency differences between generated and reflected electrical signals are monitored to detect the movement of objects within a monitored area. In practice, however, this system has been plagued by false alarms despite the introduction of filtering networks and the use of signal-isolation techniques. Furthermore, as filtering networks are added, the Doppler-type systems become extremely complex and, consequently, expensive to implement and maintain.
Examples of prior art motion detection systems include U.S. Pat. No. 3,634,884, entitled "Motion Detection System" by Vernon B. Ross; U.S. Pat. No. 3,665,443, entitled "Ultrasonic Intrusion Alarm," by Aaron A. Galvin; U.S. Pat. No. 3,742,434, entitled "Automatic Door-Opening System Using an Acoustic Object Detection System," by Warren L. Leyde and Delbert E. Marker; and U.S. Pat. No. 3,813,669, entitled "Frequency Filter Circuit Apparatus," by Leonard Thomas Saunders.